1. Field of the Invention
The present invention relates to pumping apparatus. In particular, the present invention relates to volumetric infusion pumps for administration of intravenous (IV) fluids.
2. Description of the Prior Art
To improve health care, there has been considerable effort with regard to the administration of intravenous (IV) fluid. For many years IV solutions were administered only by the force of gravity. The volume rate was measured by counting the number of drops per minute. In many instances this method proved unsatisfactory. Drop size is directly proportional to surface tension, which is affected by viscosity, temperature, type of solution, and also drop size is affected by the speed at which the drop forms. The drop rate is affected by the gravitational force and the restrictions of the tube and needle. If a tube is partly occluded, the drop rate will decrease or as the IV supply decreases the hydrostatic pressure will decrease causing a decrease in drop rate. In many cases, therefore, the variability of both the drop size and the drop rate (both of which are for the most part beyond the control of the operator) makes this method of administration of intravenous fluid unsatisfactory.
Improvements have been made by adding an elecronic drop counter together with either a controller or a peristaltic pump. The electronic drop counter and controller combination controls the drop rate but makes no improvements in controlling drop size, and also has the deficiency of not being able to control drop rate if back pressure increases beyond the hydrostatic forcing pressure. The electronic drop counter and peristaltic pump combination increases the forcing pressure but lacks an accurate metering method.
Improvement in metering methods results with the use of displacement pumps, which offer the capability of greater precison in controlling IV flow rates than is possible with the prior art IV controllers which depend on gravity. These pumps, in addition to metering the fluid, also apply positive pressure to the fluid or the IV tubing. The displacement pump can generally be categorized as peristaltic (such as described in U.S. Pat. No. 3,737,251 by Berman et al) or piston-cylinder (such as described in U.S. Pat. No. 3,985,133 by Jenkins et al) or pulsating (such as described in U.S. Pat. No. 3,874,826 by Lundquist).
The peristaltic pump, although generally an improvement over the prior art, has a number of disadvantages. First, the peristaltic action resuls in friction, which precludes administering whole blood because it causes damage to the blood cells. Second, the peristaltic action involves stretching an elastomer material, which may allow air to enter the IV solution. Third, stretching an elastomer material in a peristaltic manner does not lend itself to an efficient use of energy. Because of the ambulatory patient and possibility of a loss of power, a battery backup for the pump is required, and it is desirable therefore that the pump be as efficient as possible.
The piston-cylinder pumps of the prior art provide for accurate metering and positive pressure, but also have several disadvantages. First, because intravenous therapy requires that the pump maintain a germ-free barrier, and cost prohibits cleaning and sterilization after each use, the pumping chamber must be disposable and inexpensive to manufacture. This has been difficult to achieve with the prior art piston-cylinder pumps. To reduce manufacturing costs, some prior art pumps use only one chamber and two valves. This requires that the pump cycle have two parts, a fill, and an empty; therefore, IV therapy is interrupted during the fill portion of the cycle. Second, some of the prior art piston-cylinder pumps have difficulty meeting the requirements of providing reliable and germ-free seals against sliding surfaces. Third, the friction of a piston-cylinder pump is a cause for reduced efficiency.
The pulsating pumps provide a continuous pulsing flow, but also have significant disadvantages. First, the self-contained valving of these pumps has added to the complexity and expense of the disposable pump chamber. Second, the pulsing action against a spring load or an elastomer material does not lend itself to efficient operation.
U.S. Pat. No. 3,809,507 by Baglai describes a pump which is not intended specifically for use in IV therapy, but which does provide a continuous pulse-free flow. The valves in this pump are either located on moving parts or located at a fixed location and connected by a flexible tube. This approach does not lend itself to an economical disposable pump chamber as is required for IV applications. Also, without the valves biased or powered, the pump in the off condition may continue to supply fluid (i.e. it suffers from "siphoning"). This is an unsafe condition for IV therapy.
Another general problem with the prior art IV equipment is that no way is provided to select and sense the back pressure at which an alarm will sound. Therefore the patient, in case of an infiltration, is subjected to the maximum back pressure that the pump is able to supply. Often this pressure is beyond the safe limits for a particular patient or application.